1. Field of the Invention
The present invention relates to novel amino acid fluorides and protected amino acid fluorides and their use in synthetic biochemistry, including peptide syntheses. More particularly, this invention is directed to the N-protected amino acid fluorides, and free amino acid fluorides and the hydrogen fluoride salts thereof, the side chain of which may be unprotected or protected with a blocking group and their use thereof in peptide synthesis.
2. Background of the Prior Art
As more and more polypeptides become of medicinal importance, there is an increasing incentive to improve the methods by which they may be synthesized. In recent years, peptides which have been found to be of possible pharmacological importance include those active against various diseases, such as cancers, diabetes, and plant toxins, etc. Others have shown specific activity as growth promoters or suppressants, antibiotics, insecticides, contraceptives, anti-hypertensives, sleep-inducers, anti-depressants, analgesics, etc. The list is long and varied.
Currently, syntheses of peptides in solution by classic or various repetitive methods or on a solid support (Merrifield) are popular techniques. Solution methods have the advantages of being easily monitored and allowing purification of intermediates, if necessary, at any stage. A major drawback is the relative slow pace of the synthesis with each step being carried out manually.
The major advantage of the Merrifield Method is its easy automation so that unattended, computer-controlled machine synthesis is possible. Unfortunately, this method suffers from an inherent deficiency due to the insoluble nature of the support on which the synthesis proceeds. Unless each acylation step occurs with 100% efficiency, mixtures will inevitably be built up on the polymer. The longer the chain, the greater will be the contamination due to undesired side reactions. Products produced in such reactions remain to contaminate the desired product when at the end of the cycle it is removed from the polymeric matrix. The properties of these peptides will not differ sufficiently for peptides of greater than about 30-40 residues to make efficient separation feasible.
For very long segments (50 or more amino acids), therefore, current methods are not satisfactory. Often mixtures are obtained of such forbidding complexity that it may be difficult or impossible to isolate the desired peptide.
The problems enumerated hereinabove could be eliminated if the proper derivatives of the underlying amino acids and the proper reaction conditions could be found.
For example, FMOC, (N.alpha.-(9-fluorenylmethyl)-oxycarbonyl), protected amino acid chlorides, which are described by Carpino, et al. in J. Org. Chem. 51, 3732 (1986) have been used as acylating agents for stepwise peptide syntheses for both solution and solid phase techniques.
However, the amino acid chlorides have major drawbacks associated therewith. First, the acid chlorides react with trace amounts of water, such as moisture in the air, to give the corresponding amino acid. Therefore, they are not so stable, and as such, they are not a prime candidate for long term storage. Consequently, an objective was to find an amino acid derivative which was stable to moisture.
Moreover, another problem associated with amino acid chlorides is that it has not been possible to date to synthesize amino acid chlorides in which the protecting groups on the side chains of the amino acids can be removed under extremely mild conditions. As one skilled in the art is well aware, many of the amino acids have functional groups on the side chains which can interfere with peptide formation unless otherwise protected. In peptide synthesis, only the mildest conditions should be used to remove these protecting groups. For example, one of the easiest protecting groups to remove from the side chains containing amino, hydroxyl or carboxyl functions, such as lysine, tyrosine, threonine, serine, aspartic acid, glutamic acid and the like, is t-butyl or t-butyl containing moieties. For example, trifluoracetic acid can easily remove the t-butyl group from a serine side chain; on the other hand, a benzyl protecting group on the side chain can not be removed by said treatment but instead requires a more potent acid such as HF or trifluoromethanesulfonic acid. Therefore, the conditions for removing the benzyl group from the side chain are much harsher relative to the t-butyl groups. Furthermore, the mild catalytic hydrogenolysis of benzyl groups is not generally applicable to long chain peptides or resin attached peptides.
Although benzyl groups on the side chains of N-protected amino acid chlorides, can be prepare, such as FMOC-cysteine-S-benzyl chloride, FMOC-lysine-.epsilon. carbobenzoxy chloride, FMOC-tyrosine-O-benzyl chloride, FMOC-serine-O-benzyl chloride and FMOC aspartic acid .beta.-benzyl ester, these molecules suffer from the disadvantages described hereinabove. Consequently, an investigation was commenced to determine if t-butyl or "t-butyl like" containing groups can be used to protect the side chain of amino acid chlorides. Unfortunately, efforts in this area were unsuccessful. None of the above compounds could be synthesized if the t-butyl group was used in place of the benzyl substitution.
This was not unusual since it is well known that t-butyl-based protecting groups are readily deblocked by hydrogen chloride which is an inevitable by-product of acid chloride formation and/or long term storage (hydrolysis by trace amounts of water). For example, in the case of the FMOC-tyrosine derivative 1, the acid chloride could be obtained, but after several days it was noted to lose the t-butyl group slowly. ##STR3##
Furthermore, compound 1 as well as the analogous serine and threonine derivatives could be obtained only as oils which could not be crystallized and were therefore difficult, if not impossible, to purify.
In the case of the FMOC aspartic acid derivative 4, treatment with thionyl chloride gave only the aspartic acid anhydride 6, presumably via the unstable acid chloride 5 which undergoes intramolecular loss of t-butyl chloride. ##STR4## Lysine derivative 7 could not be converted to an acid chloride because of the marked sensitivity of the BOC function. ##STR5## Similar problems arise in the cases of Arg, His, Asn, Gln, and Trp. The net result of these problems is that only about one half of the commonly occurring amino acids can be converted to stable amino acid chlorides.
Therefore, a search was undertaken to find an amino acid candidate for use in peptide synthesis which is inexpensive, stable to moisture, and which shows great potential for long- term storage. Moreover, it was hoped that a candidate could be found where the protecting groups on the amino acid side chain could be removed under milder conditions then those used to remove the benzyl group. Preferably, it was hoped that a t-butyl containing group or a group as easily removable as t-butyl could be placed on the side chain of these amino acid candidates.
The present invention circumvents the difficulties experienced with respect to the acid chlorides and accomplishes the goals described hereinabove. The compounds of the present invention are effective in coupling with amino acids or peptides to form new peptide bonds. Moreover, the compounds of the present invention are more stable to moisture then the acid chlorides and therefore can be used for long term storage. Furthermore, t-butyl containing protecting groups and other protecting groups can be placed on the side chains of these amino acid compounds and removed under milder conditions than those required for the removal of benzyl groups. Finally, the compounds of the present invention are potent acylating agents in peptide bond formation.
These compounds are, much to our surprise, the corresponding amino acid fluorides.